A bolometer in general is a radiation detector where incoming electromagnetic radiation causes the temperature of a detector element to change in a way that can be measured and converted into an electrical output signal. Microbolometers are a special instance of bolometers with the common features that they are small in size and fabricated onto a planar semiconductor substrate using essentially the same miniature lithographic techniques that are used for manufacturing integrated circuits. An antenna-coupled microbolometer consists of a lithographically manufactured antenna that is coupled to a thermally sensitive element, which is impedance-matched to the antenna and dissipates antenna currents, thus acting as the antenna termination. If the antenna comprises two antenna branches, said thermally sensitive element is a narrow neck or isthmus that connects the antenna branches to each other. A heat bath is provided for keeping the whole antenna-coupled microbolometer in a constant temperature, so that ideally all changes of temperature in the thermally sensitive element are due to time-varying currents induced to the antenna by received electromagnetic radiation. The radiation frequencies that are to be detected with antenna-coupled microbolometers are typically between several tens of GHz and several tens of THz. Signal frequencies, i.e. the rate of change of the signal to be detected, is typically in the audio range.
A major measure of the quality of an antenna-coupled microbolometer is its Noise Equivalent Power (NEP) figure, which describes the sensitivity of the device, i.e. its ability of discriminating between an actually received signal and noise. In an ideal situation the NEP is dominated by the so-called phonon noise, which is a consequence of energy fluctuations between the thermally sensitive element and the heat bath. In order to approach such an ideal situation (the absolute value of) the responsitivity of the bolometer should be large enough. This condition is difficult to reach with conventional metal bolometers, because the absolute value of the Temperature Coefficient of Resistance (TCR) is too small for metals. Semiconductors typically have a TCR the absolute value of which is larger, but then again semiconductors are difficult to match to useful antennae, which have a typical impedance of the order of 100 ohms. A widely accepted solution is to use a superconductor film, operated at the normal metal to superconductor transition, as the thermal sensing element.
A publication J. P. Rice, E. N. Grossman, D. A. Rudman: “Antenna-coupled high-Tc air-bridge microbolometer on silicon”, Applied Physics Letters, 65(6):773–775, 1994 is known to disclose an antenna-coupled microbolometer with a NEP=9·10−12 W/√Hz at a bath temperature of 87.4 K. However, the fabrication of air-bridges of the kind shown in said publication has proven to be difficult. Additionally, making a microbolometer from a superconductor film that has a high critical temperature (so-called high-Tc superconductor) usually necessitates using a buffer layer, such as YSZ (Yttria-Stabilized Zirconia), between the superconductor film and the substrate. This increases the thermal conductivity between said materials, which is a disadvantage. Additionally microbolometers made of high-Tc superconductor films are known to suffer from excessive amounts of so-called 1/f noise, which may require using a separate optical chopper in front of the bolometer.